Summary: STS is currently evaluating several Vero Cell Assays as replacements for current animal tests; one assay involves determining the neutralizing activity of Immune Globulins(IG) on Diphtheria Toxin. With some assay modifications, lots of IG were examined for their ability to inhibit infection of Vero Cells by the current CBER Reference Smallpox vaccine, which has been shown to be active in a traditional plaque assay with several cell types. While still under development, preliminary results have indicated that current lots of IG have varying ability to protect Vero Cells in this assay.